The use of polymeric materials in the treatment of paper making fibers to improve strength is known. In the commonly assigned copending application, above-identified, for example, novel high molecular weight N-chloroamide polymers and copolymers and a process for their production by chlorination of a water-in-oil emulsions of the high molecular weight polymers are claimed.
Among the prior art polymers are the amino-aliphatic polymers set forth in U.S. Pat. No. 2,729,560 which discloses the hypochlorite treatment of acrylamide polymers whereby 20-80% of the amide groups are converted to amino groups. U.S. Pat. No. 2,890,978, on the other hand, discloses a similar hypochlorite process wherein less than 15% of the amide groups are degraded to amino groups and dry strength is imparted to paper upon application of the polymer produced. Both of these patents teach the addition of these polymers, which are the final degradation products of the Hoffmann reaction of acrylamide polymers with hypochlorite or chlorine under alkaline conditions. N-chlorinated high polymers are not obtained under such conditions.
U.S. Pat. No. 3,929,744 discloses the reaction of a sterically hindered amide polymer with a hypochlorite to produce an isocyanate functional polymer useful as a coating composition after crosslinking with a polyol or polyamine. U.S. Pat. No. 4,301,257 teaches a similar polymer produced by reacting a low molecular weight chlorinated acrylamide polymer with a tertiary amine having a pKa value of over 7 in an inert solvent. 20-100% of the amide groups of the chrge polymer are said to be converted to isocyanate groups. The maximum molecular weight of the polymer is said to be about 10,000. Again, N-chlorinated high polymers are not obtained.
Processes of making polyfunctional N-chloroamide derivatives of acrylamide homo and copolymers are taught by U.S. Pat. Nos. 4,356,289 and 4,357,447 wherein chlorine in a dilute aqueous suspension of a mineral acid at 0.degree.-40.degree. C. is employed. German Pat. No. 2,931,572 also teaches such a chlorination process. In contrast to the water soluble products produced herein, the products of these citations are not water soluble, which would not make them useful in making paper.
Belgian Pat. No. 878,978 is similar to U.S. 4,301,257, discussed above, wherein amide-containing acrylic polymers are chlorinated and rearranged to isocyanate groups in the presence of tertiary amines. The polymers are also of very low molecular weight, and are not N-halogenated.
Japanese Patent Sho No. 57-158,203 (82-158,203) discloses the reaction of polyacrylamide with a hypochlorite salt (or Cl.sub.2 and alkali) to provide isocyanate groups, then further reacted with a diamine to provide amino group-containing polymers useful for paper strengthening. These are not N-halo high molecular weight polyacrylamides.
Hahn et al. Angew, Makromol, Chem., 50, (1), 53-65, (1976) teaches the chlorination of polyacryl amides with HOCl, Me.sub.3 COCl and Cl.sub.2 O to provide chlorinated secondary amides wherein the recurring unit in the polymer is ##STR1## R.sup.5 being alkyl or aryl. They differ from the compounds of interest, especially in that they lack an -NH-group.
Water-in-oil polyacrylamide emulsions are chlorinated in U.S. Pat. No. 4,090,992 by contacting said emulsions, containing less than 40% of water, with chlorine gas at -20.degree. C. In a specific example, the emulsion is first dried and then contacted with chlorine gas, to avoid congulation or agglomeration. The mixture is then warmed and caustic soda is added. This results in the formation of an amino group containing polymer.
In the above-mentioned copending application, Ser. No. 859,681, it is disclosed that the chlorination of an acrylamide polymer in an emulsion can be accomplished without prior dehydration and is, in fact, more rapid than under dry conditions. The presence of water is postulated to be a necessary adjunct to the chlorination reaction. It has now been found possible, and is the subject matter of this invention, to halogenate in acidic solution either with halogen gas or a hypohalite. The formation of the N-haloacrylamide polymer is very rapid without degradation to amine, and it is not necessary to prepare and use emulsions. The products of the process are water soluble solutions of the polymers, and they can be used directly in the papermaking process.
It is accordingly, a principal object of the present invention to provide a process for the N-halogenation of polyacrylamides to produce N-halogenated polymers and to use them to impart wet strength to paper, paperboard and other fibrous products.
The chlorination process of this invention can be carried out on site at paper mills, thereby avoiding the storage and handling of chlorinated emulsions or solutions which are corrosive and unstable over a long period.